A brake pad set used for a disk brake of an automobile generally comprises a steel backing plate and a brake pad (friction lining layer) adhered to the backing plate. Pressing and heating powder material in which a fiber, a filler, and a binder are mixed form the brake pad. The disk brake reduces the speed of an automobile by pressing the brake pad to a steel rotor disk.
When the disk brake is operated, the disk pad set and the rotor disk may vibrate to generate squealing. This squealing becomes an unpleasant noise, so that a variety of measures have been taken so far.
In order to prevent the squealing of the brake, an inclined face can be formed on each of the both ends of a brake pad as a known measure.
FIGS. 6A-6B show a conventional brake pad set with inclined faces, in which FIG. 6A is a front view, and FIG. 6B is a bottom view. The brake pad set 1 shown in FIGS. 6A-6B comprises a steel backing plate 2 and a brake pad 3 adhered thereto. The brake pad 3 comprises a central flat friction face 3a and inclined faces 3b, 3b at both ends. Boundary lines 5, 5 between the friction face 3a and the inclined faces 3b, 3b are parallel with each other. This construction allows the lengths of the friction faces 3a contacting a rotor disk on the outer side and inner side become equal.
As described above, the provision of the inclined faces 3b, 3b allows the area of the friction face 3a to be reduced, resulting in reduced squealing.
FIGS. 7A-7B show another conventional brake pad set with inclined faces. FIG. 7A shows a brake pad set in which inclined faces 3d, 3d are formed at both ends in such a manner that a friction face 3c becomes a part of a sector of which longer circumference is positioned on the outer side of a rotor disk, and FIG. 7B shows another brake pad set in which inclined faces 3f, 3f are formed such that a friction face 3e becomes a part of a sector of which longer circumference is positioned on the inner side of a rotor disk. Boundary lines 6, 6 shown in FIG. 7A and boundary lines 7, 7 in FIG. 7B are not parallel with each other. In other words, in the brake pad set 1 illustrated in FIG. 7A, the length of the friction face 3c contacting a rotor disk on the outer side thereof is longer than that of the friction face 3c contacting a rotor disk on the inner side thereof. On the other hand, in the brake pad set 1 illustrated in FIG. 7B, the length of the friction face 3e contacting a rotor disk on the outer side thereof is shorter than that of the friction face 3e contacting a rotor disk on the inner side thereof.
It is considered that squealing of a disk brake is caused by vibration of a brake pad and a rotor disk at the operation of the brake. When the boundary lines 5, 6 are not parallel with each other, the vibration of the rotor disk can be reduced more effectively, therefore, as illustrated in FIGS. 7A-7B, the friction faces 3c and 3e have been formed in a sector shape.
By the way, the brake pad set 1 comprises the steel backing plate 2 and a brake pad attached thereto. After adhered to the backing plate 2, the brake pad is ground to finish the friction face 3a, 3c, 3e to a flat face with a desired flatness and to form the inclined faces 3b, 3d, 3f. 
The above grindings are performed with rotary whetstones 4, and a process of grinding the friction faces 3a, 3c, 3e and a process of grinding the inclined faces 3b, 3d, 3f are separately performed. For example, plurality of whetstones with different angles corresponding to the angles of the inclined faces to be ground are prepared for grinding the inclined faces 3b, 3d, 3f, and after the inclined face 3b, 3d, 3f are formed by using a predetermined tool, the friction faces 3a, 3c, 3f are ground to be flat. In addition, the grindings of both ends of the inclined faces 3b, 3d, 3f are not carried out simultaneously, but are ground from one side to the other, one by one.
As a result, with the above method, manpower is increased, and tools should be changed every time processes change from the grinding of the inclined faces to that of the friction faces, resulting in poor productivity. On the other hand, in Japanese Patent Application Laid-open No. Heisei 9-136255, a grinding method shown in FIGS. 8A-8D is proposed to grind the brake pad set shown in FIGS. 6A-6B.
At first, the brake pad set 1 is supported as shown in FIG. 8A. The rotary whetstone 4 is positioned such that a rotation shaft 4a becomes parallel to the friction face 3a. Under the condition, the rotary whetstone 4 is rotated in a direction shown by the arrow D, and grinding starts from the right end of the brake pad 3 through the rotary whetstone 4. The brake pad set 1 moves toward right in FIG. 8A, and this direction coincides with a circumference of a rotor disk not shown. As indicated by the arrow C, grinding starts from a condition that the brake pad set 1 is situated near the rotary whetstone 4, and the brake pad set 1 moves in such a manner as to become far from the rotary whetstone 4 as the brake pad set 1 moves rightward, which allows an end of the inclined face 3b to be formed as illustrated in FIG. 8B.
Then, as shown in FIG. 8B, the brake pad set 1 moves with the distance to the rotary whetstone 4 is maintained constant, which causes the flat face of the friction face 3a to be ground.
After the friction face 3a is ground, as illustrated in FIG. 8C, the brake pad set 1 is transported right while it is gradually brought close to the rotary whetstone 4 to gradually increase the amount that the friction face 3a is ground. Finally, as shown in FIG. 8D, an opposite inclined face 3b is formed. With this method, when the length of the rotary whetstone 4 along the rotation shaft 4a is designed sufficiently to cover the dimension of the brake pad set 1 only, grindings of the inclined faces 3b, 3b and the friction face 3a are carried out along a single route.
With the abovementioned method, however, following problems arise.
FIG. 9 shows an enlarged view of the brake pad set 1 and the rotary whetstone 4 illustrated in FIGS. 4A-4B. When the inclined face 3b is ground by the rotary whetstone 4, the rotary whetstone 4 moves from the position shown by the solid line along the inclined face 3b. Since the brake pad 3 is adhered to the backing plate 2, when the backing plate 2 considerably projects out of the brake pad 3, after reaching to the position shown by the phantom line, the rotary whetstone 4″ can not move along the inclined face 3b any more. Therefore, the inclined face 3b is only formed up to a contact point P of the rotary whetstone 4′ and inclined face 3b, so that a portion d remains as it is.
The remainder (grinding limit quantity) d is calculated by the following formula:d=R(1−cosα)
where, R: radius of the rotary whetstone 4, α: angle of the inclined face 3b. 
According to this formula, when R increases, d also increases; therefore, a desired inclined face 3b is liable not to be obtained.
Although shortening the radius R of the rotary whetstone 4 can solve the above problem, this measure causes the life of the rotary whetstone 4 to be shortened, and the frequency that the whetstone should be replaced will be increased, resulting in poor productivity. Further, the shorter the radius R of the whetstone becomes, the rotation of the whetstone should be faster to secure sufficient grinding velocity. Therefore, it is difficult to shorten the radius R of the rotary whetstone 4.
Further, in the above grinding method, it is impossible to form the inclined faces 3d, 3f of which boundary lines 6, 6 and 7, 7 are not parallel with each other as illustrated in FIGS. 7A-7B. It is another problem that one of the inclined faces 3d, 3f, the friction faces 3c, 3e, and the other inclined faces 3d, 3f should be ground in separate steps.